Redox shuttles are designed to prevent overcharge of a battery by shuttling, through the electrolyte, charge forced by an external circuit through a Li-ion cell without forcing intercalation/deintercalation of lithium in the electrodes of the cell. The redox shuttle is an electrolyte additive with a defined redox potential that is oxidized at the positive electrode. The oxidized species then travels back to the anode by diffusion where it is reduced back to the original species for another redox cycle. During normal operation of the cell, the redox potential of the redox shuttle is not reached and the redox shuttle molecule or ion stays inactive.
Redox shuttles have not achieved commercial use with Li-ion batteries, because current redox shuttles do not achieve the needed voltage and stability to function in high voltage Li-ion batteries, which are typically charged to 4.0 V or higher. Current redox shuttles also do not have sufficient life under overcharge conditions.
One known redox shuttle includes a closo-borate anion having the formula Li2B12F12-xHx, which is an ionic lithium salt. The chemical structure of the 2-anion is such that the boron atoms form a 12-vertex icosahedron cage with a fluorine or hydrogen atom bonded to each boron atom around the cage. These compounds show high solubility in typical Li-ion battery electrolytes and are electrochemically reversible, but some of the redox potentials, which are partially determined by the degree of substitution on the boron atom, are too high to be used in a 4.2 V cell and have limited life during overcharge. Li2B12F12, for example, may have a redox potential greater than about 4.6 V, which may be too high to be used in a 4.2 V cell. Li2B12F9H3, as another example, may have a lower redox potential than Li2B12F12, but may be more unstable, more difficult to synthesize, and more expensive than Li2B12F12. See, for example, U.S. Pat. No. 7,785,740 to Amine et al.
Prior literature (King, B. T. et. al. J. Am. Chem. Soc. Vol. 129, No. 43, 2007, 12960) has shown that closo-monocarborate anions may be reversibly oxidized and reduced by electrochemical and chemical means. However, in the context of Li-ion batteries, such carborane cage anions have been described as being fluorinated (e.g., RCB11F11). See again, U.S. Pat. No. 7,785,740 to Amine et al. Due to the high electronegativity of fluorine atoms, the fluorine-substituted closo-monocarborate anion would be expected to have a redox potential that is too high to be used in a 4.2 V cell, like the above-described fluorine-substituted closo-borate anion. In fact, because the closo-monocarborate anion has a 1-charge, the closo-monocarborate anion would be expected to have an even higher redox potential than the above-described closo-borate anion, which has a 2-charge.
The solubility of the oxidized and reduced forms of the redox shuttle is also important. A larger concentration of redox shuttle with a given diffusion constant in a given electrolyte will result in an increased current that can be shuttled. Both the oxidized and reduced forms of the redox shuttle need to have good solubility since both must diffuse though the electrolyte.
Other technologies that may be used to prevent Li-ion cell and battery overcharge include external voltage regulation and inactivation agents that are added to the electrolyte of the cell and that cause the battery to shut down if the cell is overcharged. External voltage regulation has the disadvantage that it adds to the cost and weight of the battery system while inactivation agents permanently disable the cell.